in my experience polyester is harder

you can usually just dig a finger nail into sun warmed epoxy but you never can into polyester

epoxy is the best for wood, it's easier to mix, the chemicals aren't as dangerous and the fumes nowhere near as bad

but for repairs to polyester boats polyester bonds better, is harder and cheaper

keep in mind the polyester reaction requires a "Kick" from a small toxic catalyst and that the surface chemical left over from the epoxy reaction inhibit this kick

so if someone does a small epoxy repair to a normal polyester boat but later wants to spray new gelcoat over it the gelcoat over the epoxy won't cure correctly if at all